Ah... the decision. I went through that 4 month back. : ) Ended up with the E90 M3. Mainly because the E92/E93 front seat wouldn't automatically move forward. I have a 3 month old now. So ease of moving the car seat in / out was an important criteria. The sunshades (side and back) are a great feature as well.
One thing that I wish was bigger is the trunk. With a stroller (base + bassinet), the trunk is essentially gone. Come to think of it... a Porsche sales person was trying to sell the 911 to me as a 'family and kid' friendly car. : )
I'd recommend to bring ur seats and try it out. For 4 year old, any one would work. For the 8 month, that's where your decision needs to be made IMO.
Any choice you make, as long as its an M, you'll enjoy it. : )
BTW, this was a thread I read when I was doing my research. Thought u guys may enjoy it too.
http://www.pistonheads.com/gassing/t...54339&f=48&h=0